1. Field of the Invention
The subject disclosure relates a control system for use with gas turbine engines, and more particularly, to a control system for helicopters which includes a system for preventing engine surge events by adapting acceleration schedules stored in the aircraft computer""s non-volatile memory.
2. Background of the Related Art
During the operation of a gas turbine engine, a condition known as a xe2x80x9csurgexe2x80x9d may be encountered. An engine surge is generally regarded as a mismatch between the speed of the compressor blades and the incoming air. An engine surge is typically a precursor to an engine stall event. Engine surges are characterized by a sudden and large loss of power, a loss of air flow, an increase in temperature and mechanical vibration. These mechanical vibrations, as well as the temperature increases, impose substantial stress on the engine and particularly on the turbine blades. While also occurring under other operating conditions, an engine surge event will most often occur during acceleration.
Prior attempts to ensure that engine surge events will not occur have concentrated on the establishment of fuel flow rate or acceleration schedules stored in the aircraft""s flight control computer. The acceleration schedule is traditionally provided by the engine manufacturer and is developed over time. The schedule protects the engine from surge, stall and overtemperature by regulating the fuel flow. The acceleration schedule is a function of the engine operating characteristics, and therefore, it is specific or unique to a particular engine model. The schedule typically represents the demanded rate of change of the gas generator speed (NDOTDemand) as a function of measured gas generator speed (NG) and engine inlet air temperature and pressure. The schedule is not linear, but of complex shape. The complexity of the schedule is partly due to the need to prevent the engine from operating in the compressor stall region.
Thus, for example, prior art fuel controls have been pre-programmed with an acceleration schedule and, in theory, if fuel flow is maintained in accordance with the requirements of the schedule, the engine is accelerated without surge. It is to be noted that, if an excess of fuel is delivered to the engine during surge, the engine is likely to stay in the surge condition or experience multiple surges. Therefore, prior art control systems will typically include a safety factor which is known as the surge margin. The surge margin will be taken into account in deriving the acceleration schedule and the engine will be capable of accepting a predetermined percentage of additional fuel flow before surge will occur.
Engine controls are designed and implemented for the operating characteristics of a new engine. However, the characteristics of an engine and/or its fuel metering system will vary over time as the equipment ages. Accordingly, what may have initially been an adequate acceleration schedule and/or surge margin may, with engine and/or fuel control deterioration, no longer ensure that the engine will not surge.
Commonly owned U.S. Pat. No. 4,490,791 to Morrison discloses a prior art system and method for accommodating component wear over time by modifying or adapting the acceleration schedule. In response to an engine surge the region of the acceleration schedule where the surge was encountered is lowered or decremented to increase surge margin. The disclosed system senses the decay rate of engine compressor discharge pressure during surge and modifies the pre-programmed fuel flow acceleration schedule so as to increase surge margin. Thus, an adaptive acceleration schedule is provided wherein xe2x80x9cmodifiersxe2x80x9d are stored in memory which correspond to each of the stored acceleration schedule breakpoints. The modifiers are scale factors which are initially equal to unity.
However, each time a surge is detected, the modifier which corresponds to the point on the acceleration schedule where the surge was experienced will be decremented by a preselected percentage. The fuel flow related information from the acceleration schedule is multiplied by the modifier with the result that, subsequent to a surge having been detected, future accelerations will be modified within a small corrected speed band surrounding the surge point.
Another system and method for adapting the acceleration schedules is disclosed in U.S. patent application Ser. No. 10/194,811, filed Jul. 12, 2002, entitled xe2x80x9cMethod of Engine Surge Discrimination,xe2x80x9d the disclosure of which is herein incorporated by reference in its entirety. In adapting the acceleration schedule, the disclosed system discriminates between spurious engine surges and genuine engine surges.
Although prior art systems prevent many surge events from occurring, they do so with little regard for the slow engine response which results from adapting the acceleration schedule. As a result, after an in-flight adaptation, the aircraft is typically grounded for maintenance and the engine is repaired before the aircraft is declared flight-worthy.
Therefore, there is a need for an adaptation approach which modifies the acceleration schedule to prevent future surge events and minimizes the impact on engine response time, thereby allowing the aircraft to fly until the next normally scheduled maintenance period and reducing downtime.
The disclosure of the present application relates to an engine surge avoidance system and method which adapts the acceleration schedules stored in the aircraft computer""s non-volatile memory to prevent engine surge events from occurring while minimizing reductions in engine response time. The surge avoidance system and method disclosed herein achieves this goal by adapting both the NDOT and the intercompressor (P2.5) bleed schedules in an optimum fashion.
In accordance with a preferred embodiment of the present invention, there is disclosed a method of preventing surge events in a gas turbine engine following an initial surge event. The gas turbine engine typically includes, in serial flow communication, a low pressure compressor, a high pressure compressor, a combustor, a high pressure turbine and a low pressure turbine. The method includes establishing a transient temperature limit for the gas turbine engine and estimating the combustor discharge gas temperature. Then, the combustor discharge gas temperature is compared to the established transient temperature limit. If the estimated combustor discharge gas temperature is less than the established transient temperature limit, the low pressure compressor bleed air flow rate schedule is modified so as to improve an engine surge avoidance margin. Alternatively, if the estimated combustor discharge gas temperature is greater than the established transient temperature limit, the engine fuel flow rate schedule is modified so as to improve the engine surge avoidance margin.
It is envisioned that the method disclosed herein may further include the step of measuring the combustor discharge gas temperature with a sensing means operatively positioned on the engine housing. Alternatively, the method may further include measuring a plurality of engine operating parameters and estimating the combustor discharge gas temperature using a thermodynamic engine model and the plurality of measured engine operating parameters. The plurality of operating parameters can include, but are not limited to, parameters such as component operating temperature, component inlet and exhaust air/gas temperatures or pressures, shaft, bearing speed or gear rotational speed, and engine shaft torque. The engine model is adaptive can be used to estimate component efficiencies and unknown operating parameters based on the measured operating parameters.
It is presently envisioned that prior to modifying the engine fuel flow schedule if the estimated combustor discharge gas temperature is greater than the established transient temperature limit, the maximum allowable modification to the fuel flow schedule is established and a determination is made as to whether the maximum allowable modification to the fuel flow schedule has been reached. If the maximum has been reached, the low pressure compressor bleed air flow rate schedule are modified so as to improve the engine surge margin.
The step of modifying an engine fuel flow rate schedule if the predicted combustor discharge gas temperature is greater than the established transient temperature limit includes estimating a core shaft speed for the engine.
Preferably, the low pressure compressor bleed air flow rate schedule defines a bleed valve position over an entire range of low pressure compressor shaft speeds. Additionally, the fuel flow rate schedule defines an acceleration rate for the engine""s core shaft over an entire range of engine core shaft speeds. In a representative embodiment, the acceleration rate and core shaft speed for the engine are corrected by temperature of low pressure compressor inlet air.
The present disclosure is also directed to a system for preventing surge events in a gas turbine engine following an initial surge event. As before, the gas turbine engine preferably includes in serial flow communication a low pressure compressor, a high pressure compressor, a combustor, a high pressure turbine and a low pressure turbine. A representative system includes a mechanism for establishing a transient temperature limit for a gas turbine engine, a mechanism for estimating a combustor discharge gas temperature and a device for comparing the estimated combustor discharge gas temperature to the established transient temperature limit. If the estimated combustor discharge gas temperature is less than the established transient temperature limit the system includes a mechanism for modifying the low pressure compressor bleed air flow rate schedule stored in non-volatile computer memory so as to improve an engine surge avoidance margin. If the estimated combustor discharge gas temperature is greater than the established transient temperature limit, the system includes a component for modifying an engine fuel flow rate schedule stored in non-volatile computer memory limit so as to improve the engine surge avoidance margin.
In a preferred embodiment the system further includes a sensor operatively positioned on the engine housing. Alternatively, the system includes a mechanism for measuring a plurality of engine operating parameters and estimating based on the measured parameters the combustor discharge gas temperature using a thermodynamic engine model.
Those skilled in the art will readily appreciate that the subject invention prevents engine surge events from occurring while minimizing the degradation of engine response time.
These and other unique features of the control system disclosed herein will become more readily apparent from the following description, the accompanying drawings and the appended claims.